Exception handling in the .NET Task Parallel Library with C#: reading task properties

We saw in this and this post how to catch and handle exceptions thrown by threads. A task has properties that let you read its state and determine what happened to it.

Construct two tasks:

CancellationTokenSource cancellationTokenSource = new CancellationTokenSource();
Task firstTask = Task.Factory.StartNew(() =>
{
	throw new ArgumentNullException();
});

Task secondTask = Task.Factory.StartNew(() =>
{
	cancellationTokenSource.Token.WaitHandle.WaitOne(-1);
	throw new OperationCanceledException();
}, cancellationTokenSource.Token);

cancellationTokenSource.Token.WaitHandle.WaitOne(-1) means that the task waits until it has been cancelled.

We cancel the second task using its cancellation token:

cancellationTokenSource.Cancel();

We wait for the tasks and ignore the exceptions for the sake of simplicity:

try
{
	Task.WaitAll(firstTask, secondTask);
}
catch (AggregateException)
{}

Write out a couple of properties:

Console.WriteLine("First task completed: {0}", firstTask.IsCompleted);
Console.WriteLine("First task faulted: {0}", firstTask.IsFaulted);
Console.WriteLine("First task cancelled: {0}", firstTask.IsCanceled);
Console.WriteLine(firstTask.Exception);

Console.WriteLine("Second task completed: {0}", secondTask.IsCompleted);
Console.WriteLine("Second task faulted: {0}", secondTask.IsFaulted);
Console.WriteLine("Second task cancelled: {0}", secondTask.IsCanceled);
Console.WriteLine(secondTask.Exception);

Run the code without debugging (Ctrl+F5) so that the code execution is not interrupted at the ‘throw new ArgumentNullException’ bit. You’ll see the properties printed on the console window. What do they mean?

  • IsCompleted: as the name suggests, this is ‘true’ if the task has completed
  • IsFaulted: true if the task has thrown an exception, false if it has not thrown any exceptions OR it has been cancelled
  • IsCancelled: true if the task has been cancelled
  • Exception: read of the exception throw by the task

View the list of posts on the Task Parallel Library here.

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About Andras Nemes
I'm a .NET/Java developer living and working in Stockholm, Sweden.

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